Turbidity comes from the Greek word turbid while nephelometry is the optical measurement of scattered light and is the Greek word for cloud. In water treatment plants the required detection limit is approximately one part per million of particles with a detectable change of one part per billion of particles and is read in Nephelometric Turbidity Units (NTU). The specifications required for monitoring of water is set by each country, but all are similar in their quality control characteristics and performance. The cloudiness is caused by particle concentrations in the fluid which may be organic material, clay, sand or other particulate matter. In water treatment plants this measurement provides: (1) a measurement of filter effectiveness and (2) a surrogate method for determining the level of microbial contaminates embedded in the particulate matter in the incoming raw water and any microbial remnants in the filtered water. The greater the concentration of particles in the fluid, or turbidity, the higher the level of microbial contaminates embedded in the particles and the higher the required level of disinfection procedures. Treatment water plants walk a thin line between too little disinfection in protecting the community from harmful microbes and too much disinfection causing harm from disinfection by-products. Nephelometric turbidity is the workhorse of all water treatment plants. There are essentially three ways to optically measure turbidity:                1. Forward-scatter follows Lambert-Beers Law; the light source and detector are≈180° apart;        2. Side-scatter follows Rayleigh Scattering and is an EPA nephelometric turbidity specification; the light source and detector are 90° apart;        3. Back-scatter measures reflectance of light opposite to the incident radiation; approximately 2 to 40 degrees separate the light source from the detector; the light scattering is measured as it folds back on itself.        
EPA nephelometric specifications require side-scatter measurement at 90° with mandated wavelengths between 400 nm and 600 nm (unless otherwise approved) for the light source, with no more than a 10 cm light path length. The light source must maintain a color temperature between 2200° and 3000° K. These specifications are required by EPA 180.1 Methods for federal regulations in the Determination of Turbidity by Nephlometery for water and waste water treatment plants reporting turbidity to the EPA (There is a better response at these wavelengths to smaller concentrations of turbidity in colorless water).
In order to satisfy the various wavelength requirements specified by different government agencies, a number of light sources have been approved for nephelometric turbidity measurement and reporting:
USEPA 180.1-1Incandescent2200-3000K Originai (blackbody)USEPA 180.1-2LED860 nm GLI 4-beam,USEPA 180.1-3Laser660 nm with photomultiplier tubeUSEPA 180.1-4LED525 nm (Mitchell Method M5331)USEPA 180.1-5Laser660 nm (Mitchell Method M5271)Note: the two Mitchell Methods were EPA approved and published in the Federal Register on Aug. 3, 2009.
ISO 7027 basicLED860 nm InternationalISO 7027 alternateLED550 nm International
In a review of light source literature, patents and regulations there are references to laser, LED, and incandescent for USEPA 180.1 specification of Methods and for ISO 7027 specification of Methods: USEPA 180.1-1 specifies an incandescent source of 2200-3000° K.; USEPA 180.1-2 specifies 860 nm dual sources using the same wavelength; USEPA 180.1-3 specifies 660 nm using a photomultiplier tube; ISO 7027 specifies 860 nm with an alternate 550 nm; U.S. Pat. No. 5,969,814 mentions 670 nm; U.S. Pat. No. 7,242,001 mentions 880 nm while U.S. Pat. No. 4,826,319 has 633 nm specified. U.S. Pat. No. 5,250,186 discusses the use of 633 nm and 467 nm.